[Editor] Add the ability to create/update the structure tree when saving a pdf containing newly added annotations (bug 1845087)

When there is no tree, the tags for the new annotions are just put under the root element.
When there is a tree, we insert the new tags at the right place in using the value
of structTreeParentId (added in PR #16916).

src/core/pdf_manager.js

@@ -64,6 +64,10 @@ class BasePdfManager {
     return this._docBaseUrl;
   }
 
+  get catalog() {
+    return this.pdfDocument.catalog;
+  }
+
   ensureDoc(prop, args) {
     return this.ensure(this.pdfDocument, prop, args);
   }